Laminarization of lifting surfaces, such as for example the wings of an aircraft, may provide reduced parasite drag of the wings and improvement of aerodynamic efficiency of the aircraft, wherein skin friction at laminar flow is much less than for fully turbulent flow for a given Reynolds number (Refs. 1-3). Extended laminarization, particularly of relatively thick wing sections at relatively high Reynolds numbers may be attributed to the favorable accelerating pressure gradient at the forward portion of the aerofoil and its beneficial effect on stability of the laminar boundary layer. At the lift coefficients associated with the laminar bucket, the effect of aerofoil flow laminarization produces a reduction of about 50% of the parasite drag of the aerofoil, which may potentially provide a reduction of required thrust and improved mission performance.
For unswept wings, the mechanism of laminar-turbulent transition is dominated by Tolmin-Schlihting (TS) instability of laminar boundary layers and is dependent on Reynolds number, pressure gradients, free-stream turbulence and surface quality.
While laminarization of lifting surfaces may significantly reduce parasite drag and improve aerodynamic efficiency of air vehicles, realization of this goal in the case of swept wings generally require substantial design efforts for avoiding 3-D instabilities of laminar boundary layers, including cross flow instability and attachment line transition. These instabilities may produce premature transition to turbulent flow at the forward portion of the wing sections. While some conventional NACA aerofoils comprise small leading edge radii that could be advantageous for prevention of attachment line transition when used for swept wings, these aerofoils also have reduced thickness ratio, which in turn provides relatively inferior lift-carrying capabilities.
However, for practical applications in which thick wing sections are of interest, because of physical space requirements for internal installations and storing wing internal fuel for example, aerofoils having high thickness ratio are conventionally provided with a relatively large leading edge radius, which is in conflict with principles of wing laminarization with respect to attachment line transition, if such aerofoils are used for swept wings, particularly at high Reynolds numbers.